Exhaust gas recirculation (referred to hereafter as EGR), wherein part of the exhaust gas from an engine is recycled in the air intake to reduce peak temperature and pressure in the combustion chamber, is a recognized means of reducing emission of NOx in the exhaust from direct injection diesel engines.
Although such EGR systems do reduce the amount of NOx generated, they also have the disadvantage that generation of smoke (particulate matter) tends to increase, since the recirculation of exhaust gas effectively reduces the oxygen concentration in the combustion chamber.
This smoke generation largely depends on the state of fuel injected from the fuel injection valve.
The fuel injection valves of diesel engines are generally fitted to the cylinder head at an inclined angle to avoid interference with air intake or exhaust valves. This however leads to uneven fuel injection inside the chamber so that air is used less efficiently where there is a high concentration of fuel, and more smoke is generated when exhaust gas is recycled.
In this context, a two stage fuel injection valve for diesel engines is proposed in, for example, Jikkaisho No. 61-160271 and No. 61-162572 published by the Japanese Patent Office.
In this valve, a small amount of fuel is injected in an initial lift, and after this initial amount of fuel has ignited, a larger amount of fuel is injected in a second lift. This system was intended to reduce the ignition delay in the main fuel injection, help the combustion flame to spread evenly and rapidly throughout the chamber, reduce combustion noise and emission of NOx, and generate less smoke.
However, as the fuel injection valve is fitted to the cylinder head at an inclined angle, fuel injection still tended to be uneven and led to generation of smoke when exhaust gas was recycled.
In this two stage valve, to further reduce combustion noise and emission of NOx, it has been proposed to reduce the initial lift and amount of fuel in the initial injection as far as possible.
To reduce the initial lift, the fuel flow path is made narrower. This however causes the pressure of fuel injected in the initial lift to fall, and as it is then difficult to convert the fuel to fine droplets, more smoke is again generated.
If the pressure of fuel supplied to the injection valve is increased proportionately to offset the narrowing of the fuel path, conversion of the fuel to fine droplets is improved, but it also tends to give rise to a secondary injection due to a pressure reflection wave after fuel injection is complete. This pressure reflection wave is formed in such a manner that a pressure wave generated at the fuel injection valve when it is closed is transmitted upstream and reflected at the fuel injection pump. The reflected wave is transmitted back to the fuel injection valve and pushes the valve open. In two stage valves, in particular, the pressure at which the valve opened in the initial lift (initial injection) was low, and a secondary injection therefore occurred easily when a pressure reflection wave is acting.
This secondary injection of fuel delays completion of combustion, and increases emission of unburnt components in the exhaust. It also leads to excessive fuel consumption, and severely impairs drivability and fuel cost performance of the vehicle.
Thus, even if an EGR system is combined with a two stage fuel injection valve and emission of NOx is decreased, smoke generation increases. It was therefore difficult to reduce both NOx and smoke simultaneously.